This application for an NHLBI R35 Outstanding Investigator Award is to explore the role of the plasma protein von Willebrand factor (VWF) in 2 diseases, type 2 von Willebrand disease (VWD) and sickle cell disease (SCD), and our goal is to roll 2 R01 grants covering these areas into a single, more cohesive grant with more flexibility to follow interesting scientific leads. In these two diseases, the involvement of VWF is in opposite directions, dysfunctional and with predominantly small multimers in type 2 VWD, and abundant, hyperfunctional, and large in sickle cell disease. For VWD, we will explore the molecular basis of the broad phenotypic variability associated with single mutations, testing the hypothesis that important determinants of this variability include percentage of mutant monomer incorporation into multimeric VWF and extent of VWF proteolysis by the met- alloprotease ADAMTS13. We will also explore how platelet contractility is affected in type 2 VWD, including to determine the respective contributions of plasma and platelet VWF. Finally, we will examine the intriguing possibility that in types 2A and 2B VWD a significant contribution to the hemostatic defect is provided by VWF fragments produced by excessive VWF proteolysis. In SCD, we have evidence of defective regulation of the largest VWF multimers, which are inadequately cleaved by ADAMTS13. We will investigate the basis of this lesion, and test whether a variety of measures aimed at attenuating the activity of VWF will improve the course of the disease in an animal model of SCD. Finally, we will explore the molecular basis of a fascinating observation we made in the course of studying the effect of N-acetylcysteine in SCD: that the drug induced rapid loss of very dense erythrocytes from the blood of patients. We hypothesize that the effect is caused by reduction of oxidized spectrin residues and will also explore the possibility that it may be treated even more effectively by the amino acid L-ergothioneine, which is present in mushrooms and other foods and is concen- trated in erythrocytes by a highly specific transporter. Together, these studies use state-of-the-art tools to ex- plore perplexing problems in two important human diseases, with the expectation of improved therapies for the affected patients. The R35 funding mechanism provides the flexibility to follow interesting leads discov- ered during the course of these studies, increasing the likelihood that patients will benefit.